The sigreturn() syscall is used by the signal
handling facility to atomically switch stacks, restore registers and the
thread's signal mask, and return from a signal context to resume the
processing that was interrupted by the signal.

Note that sigcontext contains machine dependent information.

Direct use of sigreturn is no longer
supported and it is not provided as a function. As used in the signal
trampoline provided by the system, if
sigreturn fails and returns then the
process is terminated.